Topic: Is Playing field supposed to be sloped

[Big Daddy]

I just got a used table off craigslist for $100.  It's in good shape and the kids an i are loving it.

But the corners are sloped and there is a slight slope towards the center from either end.  Thinking back to my foosball days, i thought the table was level with the exception of the corners.  Is this correct, or is a sloped playing field common of standard.

Any feedback is appreciated.

Thanks